I've heard some people ask a local shop to check the ukara database. Alternatively if the sale is through here have a look at the 'how did your airsoft day go' thread, if they've posted there then there's a high chance they're a player. The loadout pictures thread is also a good place to check as pictures there are often taken in game.
In general trust your gut and treat all sales like a packet of meat that's been in the fridge for a while. If it smells even slightly funky don't bother